This invention relates to motor vehicle anti-lock brake systems, and more particularly to a method of controlling rear brake pressure modulation for yaw control on split or uneven coefficient of friction road surfaces.
It is well known that activating the brakes of a motor vehicle while driving on split or uneven coefficient of friction road surfaces creates a yaw moment, possibly requiring corrective steering by the driver to maintain a desired heading. This phenomenon can be substantially eliminated by modulating the rear brake pressure based on the slip and deceleration of the rear wheel on the lower coefficient surface. However, since this strategy unnecessarily lowers the braking effort at the rear wheel on the higher coefficient surface, the vehicle stability improvement is achieved at the expense of stopping distance. Accordingly, what is needed is a brake control that provides a better balance between yaw control and braking effort.
The present invention is directed to an improved rear brake yaw control method that utilizes a measure of braking intensity to regulate both the initiation of yaw control and the braking control parameters once yaw control is initiated. The brake torque and the rate of brake pedal depression provide measures of braking intensity, and are used to develop variable slip and deceleration thresholds to which the rear wheel differential slip and differential deceleration are compared. The variable thresholds permit initiation of yaw control at relatively low levels of differential wheel slip or deceleration during panic braking, while requiring higher levels of differential wheel slip or deceleration to initiate yaw control during moderate braking. Once yaw control is initiated, the slip and deceleration thresholds are utilized to adaptively adjust the slip and deceleration targets, so that the targets are set in accordance with the slip and deceleration of the lowest coefficient rear wheel during panic braking situations, and at levels that provide more aggressive braking during moderate braking situations. In this way, yaw control is initiated and carried out in a manner that provides yaw control at a level that is suited to the braking conditions, so that vehicle stability is improved without unnecessarily increasing the vehicle stopping distance. In a preferred embodiment of the invention, the brake torque is estimated based on a periodically updated characterization of vehicle deceleration vs. brake pedal position during vehicle braking when anti-lock braking is not activated.